In chronically inflamed airway, the surface layer of epithelial cells changes composition, subepithelial matrix thickens, submucosal glands enlarge, and blood vessels change in number and caliber. These transformations contribute to airway obstruction and hypersecretion in asthma and chronic bronchitis. This project's overall goal is to test the hypothesis that extracellular proteases are determinants of remodeling in chronic airway inflammation. The proposed studies include in vitro explorations of the molecular behavior and targets of these proteases and in vivo studies of selected proteases and their targets in genetically engineered mice, including those infected with Mycoplasma pulmonis to generate a model of chronic airway remodeling. Aim 1 is to determine roles of epithelial type I membrane-anchored serine proteases, focusing on airway-expressed enzymes related to prostasin. We have identified and characterized novel members of this newly recognized family of surface-anchored peptidases, which we hypothesize to regulate salt and water content of airway fluids, protect against chronic airway infection, and maintain epithelial remodeling responses to chronic inflammation. Aim 2 is to determine roles of mast cells and their proteases in defense against chromic airway infection. Several lines of evidence from in vivo studies in mice suggest that mast cells and their proteases play critical roles in innate immune responses to infections with conventional bacteria in acute peritonitis and pneumonia. In this aim we propose to test the hypothesis that mast cells contribute to immune responses to mycoplasma, a distinct class of pathogen that produces lifelong, airway, specific infection in immunocompetent host animals accompanied by profound alterations in airway architecture. Aim 3 is to establish mechanisms and significance of variations in human tryptase genotype. In mouse and human studies, mast cell tryptases are implicated in airway remodeling in settings of allergic inflammation. We discovered striking variations in inheritance of tryptase genes, including complete absence of alpha-tryptase in about one-fifth of humans. Further, we find that humans lacking alpha-tryptases, which are catalytically inactive, inherit extra beta-tryptases, which are active. Thus, consequences of the observed variation may be profound. The studies in this Aim explore mechanisms of variation in tryptase genotype and examine roles of such variation in diseases associated with airway remodeling. Overall, the studies proposed for this project identify mechanisms of remodeling that may suggest previously unexplored strategies to prevent or reverse anatomical changes accompanying chronic airway inflammation.